•ASBR node protection—Protects interarea and Inter-AS TE label-switched paths (LSPs) from the failure of an Area Border Router (ABR) or ASBR.

•Loose path reoptimization—Allows a Multiprotocol Label Switching (MPLS) traffic engineering (TE) tunnel's LSPs to traverse hops that are not in the tunnel headend router's topology database (that is, they are not in the same Open Shortest Path First (OSPF) area, Intermediate System-to-Intermediate System (IS-IS) level, or autonomous system as the tunnel's headend router).

•Per-neighbor keys—Allows cryptographic authentication to be accomplished on a per-neighbor basis.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for MPLS Traffic Engineering: Inter-AS TE" section.

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Restrictions for MPLS Traffic Engineering: Inter-AS TE

Loose Path Reoptimization

•Midpoint reoptimization is not supported.

ASBR Forced Link Flooding

•The TE metric and affinity attributes that are known at a headend router (and used as constraints when an LSP's path is computed) are not currently signaled. Consequently, explicit router (ERO) expansions do not consider these constraints.

•Each node in an autonomous system must have a unique router ID.

•The router ID configured on a link must not conflict with the router ID within the autonomous system.

•If a link is configured for forced link flooding, the link's neighbors are not learned by regular Interior Gateway Protocol (IGP) updates. If a link is already learned about neighbors by IGP on a link, you cannot configure the link as passive. Therefore, to configure a link for forced flooding, be sure that the node does not already have a neighbor on that link.

Information About MPLS Traffic Engineering: Inter-AS TE

To configure the MPLS Traffic Engineering: Inter-AS TE feature, you need to understand the following concepts:

MPLS Traffic Engineering Tunnels

MPLS TE lets you build LSPs across your network that you then forward traffic down.

MPLS TE LSPs, also called TE tunnels, let the headend of a TE tunnel control the path its traffic takes to a particular destination. This method is more flexible than forwarding traffic based only on a destination address.

Interarea tunnels allow you to do the following:

•Build TE tunnels between areas (interarea tunnels)

•Build TE tunnels that start and end in the same area, on multiple areas on a router (intra-area tunnels)

Some tunnels are more important than others. For example, you may have tunnels carrying Voice over IP (VoIP) traffic and tunnels carrying data traffic that are competing for the same resources. Or you may simply have some data tunnels that are more important than others. MPLS TE allows you to have some tunnels preempt others. Each tunnel has a priority, and more-important tunnels take precedence over less-important tunnels.

Multiarea Network Design

You can establish MPLS TE tunnels that span multiple IGP areas and levels. The tunnel headend routers and tailend routers do not have to be in the same area. The IGP can be either IS-IS or OSPF.

To configure an interarea tunnel, use the next-address loose command to specify on the headend router a loosely routed explicit path of the LSP that identifies each ABR the LSP should traverse. The headend router and the ABRs along the specified explicit path expand the loose hops, each computing the path segment to the next ABR or tunnel destination.

Fast Reroute

MPLS Fast Reroute (FRR) is a fast recovery local protection technique that protects TE LSPs from link, shared risk link group (SRLG), and node failure. One or more TE LSPs (called backup LSPs) are preestablished to protect against the failure of a link, node, or SRLG. If there is a failure, each protected TE LSP traversing the failed resource is rerouted onto the appropriate backup tunnels.

The backup tunnel must meet the following requirements:

•It should not pass through the element it protects.

•It should intersect with a primary tunnel at a minimum of two nodes: point of local repair (PLR) and merge point (MP). The PLR should be the headend LSR of the backup tunnel, and the MP should be the tailend LSR of the backup tunnel. The PLR is where FRR is triggered when a link, node, or SRLG failure occurs.

•FRR protection can be performed for an Inter-AS tunnel only if the backup tunnel's merge point can route packets to the PLR's backup tunnel's egress interface. You can configure a static route or you can configure Border Gateway Protocol (BGP) to export the backup tunnel's egress interface to other autonomous systems.

•If the preferred link is a passive link, you must assign an administrative-weight for it. To assign an administrative weight, use the mpls traffic-eng administrative-weight command in interface configuration mode.

ASBR Node Protection

A TE LSP that traverses an ASBR needs a special protection mechanism (ASBR node protection) because the MP and PLR will be in different autonomous systems that have different IGPs.

A PLR ensures that the backup tunnel intersects with the primary tunnel at the MP by examining the Record Route Object (RRO) of the primary tunnel to see if any addresses specified in the RRO match the destination of the backup tunnel.

Addresses specified in RRO IPv4 and IPv6 subobjects can be node-IDs and interface addresses. The traffic engineering RFC 3209 specifies that you can use a router address or interface address, but recommends using the interface address of outgoing path messages. Therefore, in Figure 1 router R2 is more likely to specify interface addresses in the RRO objects carried in the resv messages of the primary tunnel (T1) and the backup tunnel.

Node IDs allow the PLR to select a suitable backup tunnel by comparing node IDs in the resv RRO to the backup tunnel's destination.

RSVP messages that must be routed and forwarded to the appropriate peer (for example, an resv message) require a route from the MP back to the PLR for the RSVP messages to be delivered. The MP needs a route to the PLR backup tunnel's outgoing interface for the resv message to be delivered. Therefore, you must configure a static route from the MP to the PLR. For the configuration procedure, see the "Configuring a Static Route from the MP to the PLR" section.

•Routers R1, R2, and R3 are in AS 100. The R1-R2 and R1-R3 links are in OSPF area 1.

•Routers R4, R5, and R6 are in AS200. The R4-R6 and R5-R6 links are in OSPF area 2.

•The link R2-R3 is in AS100, and link R4-R5 is in AS200. The links R2-R3 and R4-R5 are in OSPF area 0.

•The links R2-R4 and R3-R5 are not running an IGP because they cross the Inter-AS boundary between AS100 and AS200. Because they are not running IGP, you must configure an administrative weight for each passive interface for FRR to work. Use the mpls traffic-eng administrative-weight command in interface configuration mode.

•There is a primary tunnel, tunnel 100, from R1-R2-R4-R6.

•There is a backup tunnel, tunnel 102, from R2-R3-R5-R6.

•There is a TE tunnel, tunnel 101, from R6-R5-R3-R1 for returning data traffic for tunnel 100.

•There is a TE tunnel, tunnel 103, from R6-R5-R3-R2 for returning data traffic for tunnel 102.

•The explicit paths of all the tunnels use loose hops.

•The R2-R4 link is configured to be link flooded in both R2's and R4's IGP. The R3-R5 link is configured to be link flooded in both R3's and R5's IGP.

Router R2 needs to ensure the following:

•Backup tunnel intersects with the primary tunnel at the MP, and therefore has a valid MP address. In Figure 1, R2 needs to determine that tunnel 100 and backup tunnel 102 share MP node R6.

•Backup tunnel satisfies the request of the primary LSP for bandwidth protection. For example, the amount of bandwidth guaranteed for the primary tunnel during a failure, and the type of protection (preferably protecting against a node failure rather than a link failure).

Node-IDs Signaling in RROs

ASBR node protection includes a node-ID flag (0x20), which is also called a node-ID subobject. When it is set, the flag indicates that the address specified in the RRO object in the resv message is the node-ID address. The node-ID address refers to the traffic engineering router ID.

A node must always use the same address in the RRO (that is, it must use IPv4 or IPv6, but not both).

To display all the hops, enter the following command on the headend router. Sample command output is as follows:

Router(config)# show ip rsvp reservations detail

Reservation:

Tun Dest: 10.10.0.6 Tun ID: 100 Ext Tun ID: 10.10.0.1

Tun Sender: 10.10.0.1 LSP ID: 31

Next Hop: 10.10.1.2 on Ethernet0/0

Label: 17 (outgoing)

Reservation Style is Shared-Explicit, QoS Service is Controlled-Load

Average Bitrate is 10K bits/sec, Maximum Burst is 1K bytes

Min Policed Unit: 0 bytes, Max Pkt Size: 0 bytes

RRO:

10.10.0.2/32, Flags:0x29 (Local Prot Avail/to NNHOP, Is Node-id)

10.10.4.1/32, Flags:0x9 (Local Prot Avail/to NNHOP)

Label subobject: Flags 0x1, C-Type 1, Label 17

10.10.0.4/32, Flags:0x20 (No Local Protection, Is Node-id)

10.10.7.1/32, Flags:0x0 (No Local Protection)

Label subobject: Flags 0x1, C-Type 1, Label 17

10.10.0.6/32, Flags:0x20 (No Local Protection, Is Node-id)

10.10.7.2/32, Flags:0x0 (No Local Protection)

Label subobject: Flags 0x1, C-Type 1, Label 0

Resv ID handle: 0100040E.

Status:

Policy: Accepted. Policy source(s): MPLS/TE

For a description of the fields, see the Cisco IOS Quality of Service Solutions Command Reference.

Addition of the Node-ID Subobject

When a fast reroutable LSP is signaled, the following actions occur:

•An LSR adds a node-ID subobject and an incoming label subobject in the resv message.

•If there is an RRO object in the path message, an LSR adds a node-ID subobject, an RRO IPv4 subobject that records the interface address, and an incoming label subobject in the resv message.

If you enable record-route on the headend LSR, the interface addresses for the LSP are included in the RRO object of the resv message.

To enable record-route, enter the following command with the record-route keyword:

tunnel mpls traffic-eng record-route

Processing of an RRO with Node-ID Subobjects

The node-ID subobject is added to the RECORD_ROUTE object before the label route subobject. If RECORD_ROUTE is turned on, the RRO object consists of the following in this order: node-ID, interface address, and label.

Merge Point Location

The destination of the backup tunnel is the node-ID of the MP. A PLR can find the MP and appropriate backup tunnel by comparing the destination address of the backup tunnel with the node-ID subobjects included in the resv RRO for the primary tunnel.

When both the IPv4 node-ID and IPv6 node-ID subobjects are present, a PLR can use either or both of them to find the MP address.

Determination of Backward Compatibility

To remain compatible with nodes that do not support RRO IPv4 or IPv6 node-ID subobjects, a node can ignore those objects. Those nodes cannot be the MP in a network with interarea or Inter-AS traffic engineering.

Loose Path Reoptimization

Interarea and Inter-AS LSPs

If the LSP of an MPLS TE tunnel traverses hops that are not in the headend router's topology database (that is, the hops are in a different OSPF area or IS-IS level), the LSP is called an interarea TE LSP.

If the LSP of the tunnel traverses hops that are in a different autonomous system (AS) from the tunnel's headend router, the LSP is called an Inter-AS TE LSP.

Interarea LSPs and Inter-AS TE LSPs can be signaled using loose hop subobjects in their EROs. The headend does not have "strict" knowledge of hops beyond its area, so the LSP's path is "loosely" specified at the headend. Downstream routers processing these loose hop subobjects (which do have the knowledge) are relied upon to expand them into strict hops.

Loose Hop Configuration

Beyond the headend area, configure hops as loose hops. Typically you specify only the ABRs and the tailend router of a tunnel, but any other combination is allowed.

Loose Hop Expansion

Loose hop expansion is the conversion of a single ERO loose hop subobject into one or more strict hop subobjects.

Interarea and Inter-AS TE LSPs can be signaled using loose hop subobjects in their EROs. When a router receives a path message containing an ERO that has a loose hop as the next address, the router typically expands the ERO by converting the single loose hop subobject into one or more strict hop subobjects. The router typically has the knowledge, in its topology database, of the best way to reach the loose hop and computes this path by using constraint-based shortest path first (CSPF). So the router substitutes this more specific information for the loose hop subobject found in the ERO. This process is called loose hop expansion or ERO expansion.

Loose hop expansions can occur at one or more hops along an LSP's path. This process is referred to as loose path reoptimization.

Tunnel Reoptimization Procedure

Tunnel reoptimization is the signaling of an LSP that is more optimal than the LSP a TE tunnel is currently using (for example, it may be shorter or may have a lower cost), and the switching over of the tunnel's data to use this new LSP.

The new more optimal TE LSP is always established and the data moved onto it before the original LSP is torn down (so it is called the "make before break" procedure). This ensures that no data packets are lost during the transition to the new LSP.

For tunnel reoptimization to function:

•Each router must be configured with the mpls traffic-eng reoptimize events link-up command.

•Each passive link must have an assigned administrative weight. To configure an administrative weight, use the mpls traffic-eng administrative-weight command in interface configuration mode.

The TE LSPs reoptimization process is triggered under the following circumstances:

Regardless of how reoptimization is triggered, the headend router reoptimizes a tunnel only if it can find a better path than the one the tunnel currently uses. If there is not a better path in the local topology database, no new LSP is signaled and reoptimization does not occur.

Prior to the addition of loose path reoptimization, interarea TE LSPs were not reoptimized if a better path became available in any area beyond the headend area. This is because the headend router was not capable of finding a better path when the better path existed in an area beyond its view (that is, it was not in its local topology database).

With the addition of loose path reoptimization, a tunnel's headend can reoptimize LSPs even if they span multiple areas, levels, or autonomous systems. This is done via the implementation of a query and response protocol defined in draft-vasseur-mpls-loose-path-reopt-02.txt. This draft defines a protocol whereby a tunnel's headend may query downstream routers to perform ERO expansion for this tunnel's LSP. These downstream routers respond in the affirmative if they can find a more optimal path than the one in use. (This is done via a new ERO expansion.) Having received an affirmative answer to its query, a headend signals a new LSP for the tunnel, and the new LSP benefits from a new ERO expansion along the better path.

Loose path reoptimization is on by default, and cannot be disabled. Whenever an LSP reoptimization is attempted but the headend fails to find a better path, if the LSP contains loose ERO subobjects, a query is sent downstream to determine whether downstream routers can find a better path. If an affirmative answer comes back, the LSP is reoptimized. That is, a new LSP is signaled (which will follow the better path), the tunnel's data packets are switched over to use this new LSP, and the original LSP is torn down.

For details on this query and response protocol, see draft-vasseur-mpls-loose-path-reopt-02.txt.

ASBR Forced Link Flooding

When you configure forced link flooding on an interface, the MPLS TE link management module advertises the link to all nodes. As a result of this advertisement, the TE topology database on all the nodes within the Inter-AS is updated with this information.

ASBR forced link flooding allows the links to be advertised even if IGP adjacencies are not running over these links. TE LSPs can traverse these links at the edge of a network between two nodes running BGP (or static routes) even if the exit ASBR is not listed in the IP explicit path. Therefore, a headend LSR can consider that link when it computes its TE LSP path.

If a multiaccess link has more than one neighbor, a Type 10 LSA is advertised for each neighbor. In the topology database, neighbors are represented by point-to-point neighbor relationships.

Link TLV

A link TLV describes a single link and contains multiple sub-TLVs.

An opaque LSA contains a single link TLV.

For each ASBR-to-ASBR link, an ASBR must flood an opaque LSA containing one link TLV that has the link's attributes.

A link TLV comprises the following sub-TLVs:

•Link type (1 octet)—(Required) Defines the type of the link. The link type of a passive interface always is 1 (point-to-point), even for a multiaccess subnetwork.

•Link ID (4 octets)—(Required) Identifies the other end of the link for a point-to-point link. Includes the system ID of the neighbor, requires static configuration for a multiaccess ASBR-to-ASBR link, and includes the system ID of the neighbor.

•Remote interface IP address (4 octets)—Specifies the IP addresses of the neighbor's interface corresponding to this link. The remote interface IP address is set to the router ID of the next hop. There must be a static configuration for the ASBR-to-ASBR link.

•Traffic engineering metric (4 octets)

•Maximum bandwidth (4 octets)

•Maximum reservable bandwidth (4 octets)

•Unreserved bandwidth (32 octets)

•Administrative group (4 octets)

IS-IS TLV

In IS-IS, when autonomous system A1 floods its LSP, it includes the system ID and a pseudonode number.

If three autonomous systems are connected to a multiaccess network LAN, each link is considered to be a point-to-point link. The links are marked with the maximum metric value so that the inter-ASBR links are considered by CSPF and not by shortest path first (SPF).

TE uses the protocol TLV type 22, which has the following data structure:

•System ID and pseudonode number node (7 octets)

•Default metric (3 octets)

•Length of sub-TLVs (1 octet)

•Sub-TLVs (0 to 244 octets), where each sub-TLV consists of a sequence of the following: 1 octet for subtype, 1 octet for the length of the value field of the sub-TLV, and 0 to 242 octets for the value

IPv4 address for a neighboring router on this link. This will be set to the router ID of the next hop.

9

4

Maximum link bandwidth.

10

4

Reservable link bandwidth.

11

32

Unreserved bandwidth.

18

3

TE default metric.

250 to 254

—

Reserved for Cisco-specific extensions.

255

—

Reserved for future expansion.

Note The TE router ID is TLV type 134.

Topology Database

When the topology database module receives a link-state advertisement (LSA), the module scans the LSA to find the neighbors of the links. The ASBR link is part of the same LSA and is installed in the TE topology database like any other link.

During the CSPF operation, the TE headend module uses the TE topology database to find a path to the destination. Because the Inter-AS links are part of the TE topology database, the CSPF operation uses these links to compute the LSP path.

Link Flooding

The IGP floods information about a link in the following situations:

•When a link goes down

•When a link's configuration is changed (for example, when the link cost is modified)

•When it is time to periodically reflood the router's IGP information

•When link bandwidth changes significantly

Flooding is a little different in IS-IS and OSPF. In OSPF, only information about the link that has changed is flooded, because a Type 10 LSA contains a single link advertisement. In IS-IS, information about all links on a node is flooded even if only one has changed, because the Type 22 TLV contains a list of all links on the router.

How to Configure MPLS Traffic Engineering: Inter-AS TE

This section contains the following procedures for configuring MPLS Traffic Engineering: Inter-AS TE:

Enters the subcommand mode for IP explicit paths and creates or modifies the explicit path. This command places the router in IP explicit path configuration mode.

Step 4

next-address looseA.B.C.D

Example:

Router(cfg-ip-expl-path)# next-address loose 10.10.0.2

Specifies the next loose IP address in the explicit path. Each area border router (ABR) the path must traverse should be specified in a next-address loose command. This command places the router in global configuration mode.

Step 5

interface tunnelnumber

Example:

Router(config)# interface tunnel 100

Configures a tunnel interface. This command places the router in interface configuration mode.

Step 6

tunnel mpls traffic-eng fast-reroute

Example:

Router(config-if)# tunnel mpls traffic-eng fast-reroute

Enables an MPLS traffic engineering tunnel to use an established backup tunnel in the event of a link failure.

Configuring Loose Hops: Examples

Configuring an Explicit Path on the Tunnel That Will Cross the Inter-AS Link: Example

The following commands configure a loose IP explicit path named route1 suitable for use as a path option with Inter-AS TE with the destination 10.10.10.6 that is to traverse ABRs 10.10.0.2 and 10.10.0.4. The tunnel headend and the specified ABRs will find a path from the source AS100 to the destination 10.10.0.6 in AS200. See Figure 1.

Router(config)# ip explicit-path name route1 enable

Router(cfg-ip-expl-path)# next-address loose 10.10.0.2

Router(cfg-ip-expl-path)# next-address loose 10.10.0.4

Router(cfg-ip-expl-path)# next-address loose 10.10.0.6

Note that the explicit path for an interarea TE tunnel need not specify the destination router because the tunnel configuration specifies it in the tunnel destination command. The following commands configure an explicit path named path-without-tailend that would work equally well for the interarea tunnel created in the previous example:

Router(config)# ip explicit-path name path-without-tailend

Router(cfg-ip-expl-path)# next-address loose 10.10.0.2

Router(cfg-ip-expl-path)# next-address loose 10.10.0.4

Configuring a Route to Reach the Remote ASBR in the IP Routing Table: Example

In the following example, packets for the ASBR whose router ID is 10.10.0.1 will be forwarded via tunnel 101:

Router> enable

Router# configure terminal

Router(config)# ip route 10.10.0.1 255.255.255.255 tunnel 101

Configuring a Static Route from the MP to the PLR: Example

In the following example, a static route is configured from the MP to the PLR. The outgoing interface is tunnel 103.

Router> enable

Router# configure terminal

Router(config)# ip route 10.10.3.1 255.255.255.255 tunnel 103

Configuring ASBR Forced Link Flooding: Examples

This section includes the following ASBR forced link flooding examples:

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Feature Information for MPLS Traffic Engineering: Inter-AS TE

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.

Cisco IOS software images are specific to a Cisco IOS software release, a feature set, and a platform. Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/cfn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Note Table 2 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release. Unless noted otherwise, subsequent releases of that Cisco IOS software release also support that feature.

In 12.2(33)SRA, the nbr-if-addr keyword was added to the mpls traffic-eng passive-interface command.

In 12.2(33)SRB, support was added for SSO recovery of LSPs that include loose hops.

In 12.2(33)SXH, this feature was integrated into Cisco IOS Release 12.2(33)SXH.

In 12.4(20)T, this feature was integrated into Cisco IOS Release 12.4(20)T.

Glossary

ABR—Area Border Router. A routers connecting two areas.

adjacency—The MPLS TE Forwarding Adjacency feature allows a network administrator to handle a traffic engineering, label-switched path (LSP) tunnel as a link in an Interior Gateway Protocol (IGP) network based on the Shortest Path First (SPF) algorithm. A forwarding adjacency can be created between routers regardless of their location in the network. The routers can be located multiple hops from each other.

area—A logical set of network segments (for example, one that is OSPF-based) and their attached devices. Areas usually are connected to other areas by routers, making up a single autonomous system. OSPF and IS-IS define their areas differently. OSPF area borders are marked by routers. Some interfaces are in one area, and other interfaces are in another area. With IS-IS, all the routers are completely within an area, and the area borders are on links, not on routers. The routers that connect the areas are level-2 routers, and routers that have no direct connectivity to another area are level-1 routers.

ASBR—Autonomous System Boundary Router. The router is located between an OSPF autonomous system and a non-OSPF network. ASBRs run both OSPF and another routing protocol, such as RIP. ASBRs must reside in a nonstub OSPF area.

autonomous system—A collection of networks under a common administration sharing a common routing strategy. Autonomous systems are subdivided by areas.

backup tunnel—An MPLS traffic engineering tunnel used to protect other (primary) tunnel's traffic when a link or node failure occurs.

border router—A router at the edge of a provider network that interfaces to another provider's border router using extended BGP procedures.

Cisco Express Forwarding—A means for accelerating the forwarding of packets within a router, by storing route lookup information in several data structures instead of in a route cache.

Fast Reroute—A mechanism for protecting MPLS traffic engineering (TE) LSPs from link and node failure by locally repairing the LSPs at the point of failure, allowing data to continue to flow on them while their headend routers attempt to establish end-to-end LSPs to replace them. FRR locally repairs the protected LSPs by rerouting them over backup tunnels that bypass failed links or nodes.

flooding—A traffic-passing technique used by switches and bridges in which traffic received on an interface is sent out all the interfaces of that device except the interface on which the information was received originally.

Inter-AS LSP—An MPLS traffic engineering label-switched path (LSP) that traverses hops that are not in the headend's topology database (that is, it is not in the same OSPF area, IS-IS area, or autonomous system as the headend).

interface—A network connection.

IP explicit path—A list of IP addresses, each representing a node or link in the explicit path.

IS-IS—Intermediate System-to-Intermediate System. OSI link-state hierarchal routing protocol based on DECnet Phase V routing, where intermediate system (IS) routers exchange routing information based on a single metric to determine the network topology.

link—A point-to-point connection between adjacent nodes.

LSA—link-state advertisement. A broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables.

LSP—label-switched path. A configured connection between two routers, in which MPLS is used to carry packets. An LSP is a path created by the concatenation of one or more label-switched hops, allowing a packet to be forwarded by swapping labels from an MPLS node to another MPLS node.

midpoint—A transit router for a given LSP.

midpoint reoptimization—Ability of a midpoint to trigger a headend reoptimization.

MP—merge point. The LSR where one or more backup tunnels rejoin the path of the protected LSP, downstream of the potential failure. An LSR can be both an MP and a PLR simultaneously.

MPLS—Multiprotocol Label Switching. Packet-forwarding technology, used in the network core, that applies data link layer labels to tell switching nodes how to forward data, resulting in faster and more scalable forwarding than network layer routing normally can do.

multicast—Single packets are copied by the network and sent to a specific subset of network addresses. These addresses are specified in the Destination address field. (Multicast is an efficient paradigm for transmitting the same data to multiple receivers, because of its concept of a Group address. This allows a group of receivers to listen to the single address.)

node—Endpoint of a network connection or a junction common to two or more lines in a network. Nodes can be interconnected by links, and serve as control points in the network.

passive link—When IGP is not running on the link between two ASBRs, traffic engineering informs the IGP to flood link information on behalf of that link (that is, it advertises that link).

PLR—point of local repair. The headend LSR of a backup tunnel.

router—A network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.

RSVP—Resource Reservation Protocol. An IETF protocol used for signaling requests (setting up reservations) for Internet services by a customer before that customer is permitted to transmit data over that portion of the network.

SPF—shortest path first. A routing algorithm used as the basis for OSPF operations. When an SPF router is powered up, it initializes its routing-protocol data structures and then waits for indications from lower-layer protocols that its interfaces are functional.

SRLG—Shared Risk Link Group. Sets of links that are likely to go down together (for example, because they have the same underlying fiber).

tailend—The router upon which an LSP is terminated. This is the last router in the LSP's path.

TE—traffic engineering. The techniques and processes used to cause routed traffic to travel through the network on a path other than the one that would have been chosen if standard routing methods had been used.

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Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.